This invention relates to a basestation for a cellular communication system, and in particular to a method for achieving a required accuracy for the frequencies of signals generated within the basestation.
In common with other radio frequency transmitter and receiver devices, a cellular basestation needs to be able to generate signals having a desired frequency, with an acceptable level of accuracy. These signals can then be used in order to generate the signals that are in fact transmitted over the air interface to other devices. Similarly, these signals can also be used in demodulating signals received over the air interface from other devices.
In order that the signals transmitted by the basestation can be successfully demodulated by their respective receivers, and in order that the basestation can successfully demodulate its received signals, the frequencies of these signals need to be controllable with a high degree of accuracy. For example, the 3GPP specification requires that an oscillator in a basestation should be accurate to +/−0.1 ppm.
The required degree of accuracy can typically be obtained by using an oscillator, such as an Oven Controlled Temperature Compensated (OCXO) crystal oscillator. However, this has the disadvantage that such oscillators are somewhat expensive, in particular relative to the overall cost of a basestation that may only intended be to provide coverage over a relatively small area.